Sunday, March 4, 2018

Fly for small tent (part 2)

Based on a guy line test fit, guying at the corners is mostly sufficient.

However, the long sides have a tendency to touch the side of the tent.  Adding guys at the middle of each of the long sides seems to resolve this issue.  The only subtlety is the door; the guy line should attach to the left side of the door zipper (when facing the tent) since I'm right-handed.

Parts needed
  • 6 stakes
  • 6 guy lines each about 2 feet long
  • 4 corner attachments
  • 2 side attachments for the long sides
  • zipper pull
The top of the tent -- where all the seams come together -- is something of a mess.  To fix that, I sewed a rectangular patch at the top.

The six guy point attachments are made from siezed loops of 1/8" braided nylon cord.

These loops are sewn directly into the corners...

... the middle of the back ...

... and the left side of the zipper.

The zipper pull is a bit short, so I added a lanyard.  The lanyard starts as 2 feet of 1/8" nylon cord.

Here is the completed lanyard, which is tied directly onto the zipper pull.

Finally, I added guy lines and set the tent up.  Once up, I sealed the seams!

Sunday, February 18, 2018

Fly for a small tent (parrt 1)

We have two small, inexpensive tents that we purchased years ago.  They're simple and easy to set up.  Best of all, they're very lightweight.

But sadly, the rain fly is way too small.  Since it seems that no really suitable tents are on the market, I set about making a replacement fly.

Based on some quick measurements of the tent, the basic idea is that there are four panels.  The front panel is split with a zipper.

The panels lay out on about 6 yards of fabric on a 60" wide roll.

Here's the bill of materials:
  • 6 yd of ripstop waterproof nylon.  I'm using 1.1 oz weight, which should lead to about a total of 7 oz.
  • 3 foot zipper
  • seam sealer
  • 4 peg tiedowns

First, I chalked out all of the pieces on the fabric to make sure that everything fit nicely.
The front panel (with the zipper) is handled first so that I can compensate for any issues with the front panel size.  The zipper seam is a little complicated.

I figure that if the overlap (v above) is more than three times the zipper half width (z above), it will be fit nicely.  This means that the seam allowance ought to be about 2 v + 1.5 z.  For my zipper, this was around 5 inches.

Here are the two panels to be zipped together.

Forming the pocket for the zipper.

Sewing the zipper into its pocket.

Completed seam.

The pocket folds over the zipper and is sewn in place.

Duplicated on the back as well.

The zipper doesn't open all the way to the top, so the front and back pockets are pinned together to close the front panels together.

Completed zipper pocket.

Since ripstop nylon is slippery stuff, each of the main seams is sewn in two steps.  (I was trying this seam as an alternative to the usual felled seam.  I did three of the main seams as below, and the last as a felled seam.  The felled seam was much easier, but perhaps wasn't as well aligned.)

First, the fabric is pinned...  (use lots of pins!  the fabric is quite prone to sliding!)

... stay-stitched flat...

... then folded, re-pinned, and final stitched.

Here is the tent with the fly at this point. 

The fly is a bit floppy, but this is not surprising.  Shaping still needs to be done, and guying is critical. The attachment points for the guys are determined experimentally with the tent pitched.

Still to come:
  • Guy lines
  • Stitch the bottom edge
  • Shape the top
  • Clean up loose thread ends
  • Seal the seams
  • Add a nice zipper pull

Saturday, January 27, 2018

Saxaphone ligature screw

Edwin's saxaphone ligature takes two 6-32 screws.  One of them broke, so I made a replacement.  I started with 1/2" diameter steel rod.  I think it was zinc plated, but that wasn't too critical; whatever plating was originally there was removed.

I started by turning the portion to be threaded to 1/8".  Since the length wasn't too critical, I just used the good screw to figure the length.

I threaded the end by hand.  (I could have used a geometric die, but the setup would have been longer...)

Once threaded, I turned the shoulder that separates the threaded portion from the handle.

The shoulder actually tapers a bit, which I did with the graver.

The handle is a rounded ball, which I shaped with the graver after using a parting tool to remove most of the waste.

Here is the part next to the broken screw after parting.

I removed the leftover spigot by reversing the screw in the lathe -- good thing I have an 1/8" collet.

Now the handle is ready for final shaping.

Since I had to remove lots of material, I figured I could either spend time with a file or the bench grinder.  The bench grinder seemed like the better option (it may not have been any faster, actually).  I gripped the part in a pin vice so that I was securely in hand.

I ground both flats with numerous breaks to dunk in cool water...

After polishing, hardening, tempering, and final polishing, here is the final product in place.

The handle is a little wider than the original, but it does the job.

Monday, January 15, 2018

80 meter antenna

We built several Cricket 80a kits, which operate in the 80 meter amateur band.  Since we don't have any antennas specifically for this band, I decided to make one.  The design is a basic dipole fed with coax and a current balun at the feedpoint.

Here are the feedpoint parts: 

The coil is bifilar wound with 13 turns of 22 AWG speaker wire through a toroid.  It measures around 350 microhenries.  The extra loop is for a support cable to lift the feedpoint.  The enclosure is a watertight plastic container that has holes drilled for the SO-239 connector and mounting hardware.  I found that it was easier to control the drilling by hand (rather than by power drill), and a step drill made the holes cleanly deburred.

The two wires attach to the coax center and shield, while the other leads attach to the antenna.  It made sense to do the electrical work first...

... and then install it in the container.  I applied plastic epoxy to each of the pass-throughs before installing hardware in an attempt to seal out any water.  After the hardware was installed, I screwed in the antenna connections tight.

Here is the final enclosure, ready for the radiating wires.

Connecting from the hooks to the radiating wire segments is done with a short feeder segment of stranded wire.  Loops are soldered in the stranded wire while it's installed on the enclosure.  To avoid melting the plastic, I gripped a hemostat onto the plastic side of the junction to draw the heat.

Here are the feeder segments ready for the radiating wire segments. 

The next step was to design the radiating wire segments.

For the radiating structure, we are constrained by the feedline length (25 feet of RG-58) and the confines of our lot.  We're lucky that the antenna basically runs the length of one side of our property, basically touching the ground on one end due to ground slope.  Here is what NEC seems to suggest:
  • Height of antenna above ground: 2 meters
  • Length of each leg: 20 meters = 65 feet
Here is the impedance and VSWR according to NEC:

Here is the radiation pattern according to NEC, which clearly indicates that the main beam is vertical, which should be good for near vertical incidence skywave (superimposed on the antenna structure):

Given this plan, I measured out two runs of 67 feet each; better to cut long and trim than the other way around.

The wires are attached to the feeders.

Ready for installation!

Then I strung the works into position.  This took a while; after trimming off the excess wire, I got a near perfect SWR around 3.560 - 3.580 MHz, right where I wanted it.  As NEC predicted, the antenna seems to degrade higher in the band, with a 2.5:1 SWR around 3.800 MHz.